The Metropolitan Police Service announced in an early Saturday memo that the fatal stabbing in the Essex town of Leigh-on-Sea has been declared a terrorist incident.
As a result, the probe has been turned over to the Met's Counter Terrorism Command. Command investigators will be working alongside the Eastern Region Specialist Operations Unit (ERSOU) and the Essex Police.
"At the scene, officers found a man with multiple stab wounds. He was given emergency medical treatment by emergency services, but sadly died at the scene," the Met memo detailed, confirming Amess as the victim.
Specialist officers have been tasked with supporting Conservative MP's family.
A 25-year-old British man remains in Essex police custody after being arrested at the scene, authorities confirmed. Two London addresses are being searched by officers as part of the Counter Terrorism Command investigation.
Investigators believe the suspect acted alone, but additional information is still being sought.
Home Secretary Priti Patel has referred to the stabbing incident as "an attack on democracy" and, on Friday, chaired a meeting with UK police, intelligence and security agencies.
"The home secretary has asked all police forces to review security arrangements for MPs with immediate effect and will provide updates in due course," said a spokesperson for Patel.
A number of politicians and other prominent UK officials have paid tribute to Amess, including UK Prime Minister Boris Johnson, who described the MP as "one of the kindest, nicest, most gentle people in politics."